JNTU
JNTU B.Tech III HEAT TRANSFER Previous Question Paper (August/September - 2021 - R18)
Answer any five questions. All questions carry equal marks.
Main
What is the significance of different modes of heat transfer? Explain with suitable examples.
Answer
Heat transfer occurs through three fundamental modes, each significant in engineering design and thermal analysis:
- Conduction: Heat transfer through a solid or stationary fluid due to a temperature gradient. It is significant in determining the thermal resistance of materials. Example: Heat flow through a furnace wall or a metal rod heated at one end.
- Convection: Heat transfer between a solid surface and a moving fluid. It is critical for cooling systems and heat exchangers. Example: Heat removal from an engine block by circulating coolant or air cooling of electronic components.
- Radiation: Heat transfer via electromagnetic waves, requiring no medium. It is the dominant mode at high temperatures or in vacuum conditions. Example: Heat transfer from the sun to the earth or heat loss from an uninsulated steam pipe to the surroundings.
A certain building wall consists of 0.25 m of concrete (k = 1.8 W/m K), 0.05 m of fiber glass insulation and 15 mm of gypsum board (k = 0.03 W/m K). The inside and outside convection coefficient is 15 W/m2 K and 40 W/m2 K respectively. The outside air temperature is – 8 0C and the inside temperature is 28 0C. Calculate the overall heat transfer coefficient for the wall, the R value, and the heat loss per area.
Answer
Given: L1 = 0.25 m (concrete), k1 = 1.8 W/mK; L2 = 0.05 m (fiberglass), k2 = 0.03 W/mK; L3 = 0.015 m (gypsum), k3 = 0.5 W/mK (assumed typical); h_in = 15 W/m2K; h_out = 40 W/m2K.
The total thermal resistance per unit area (R_total) is:
The overall heat transfer coefficient (U) is:
The heat loss per unit area (q") is:
Explain the need of boundary conditions in solving heat conduction problems? Discuss different types of boundary conditions.
Answer
Sign in to view the complete answer paper.
A long hollow cylinder has its inner and outer surfaces maintained at temperatures Tb and Ta respectively. The inner and outer radii are b and a respectively. Calculate the temperature profile in the solid section of the cylinder and determine the flux at both surfaces. Assume steady state condition.
Answer
Sign in to view the complete answer paper.
A hair dryer may be idealized as a circular duct through which a small fan draws ambient air and within which the air is heated as it flows over a coiled electric resistance wire. If a dryer is designed to operate with an electric power consumption of 1000 W and to heat the air from an ambient temperature of 25 0C to a discharge temperature of 50 0C, at what volumetric flow rate should the fan operate? Heat loss from the casing to the ambient air and the surroundings may be neglected. If the duct has a diameter of 75 mm, what is the discharge velocity of the air? The density and specific heat of the air is 1.1089 kg/m3 and 1005 J/kg K.
Answer
Sign in to view the complete answer paper.
A brass rod in the form of a fin 100 mm long and 5 mm in diameter extends horizontally from a casting which is at 2000C. The air temperature is 200C and provides a heat transfer coefficient of 30 W/m2-K. What is the heat transfer from the rod? Evaluate the temperature of the rod at 50 mm from the base and at the free tip. Now, if this fin is replaced by two identical fins of 50 mm length. All other parameters and dimensions remain the same. What is the heat transfer from this combination? Evaluate the temperature of the fin at the tip.
Answer
Sign in to view the complete answer paper.
Under what conditioning a small mass can be considered as lumped body for unsteady state condition? Explain.
Answer
Sign in to view the complete answer paper.
Engine oil at 400C flows with a velocity of 1 m/s over a 2 m long plate whose surface is maintained at uniform temperature of 800C. Determine the local and average heat transfer coefficients.
Answer
Sign in to view the complete answer paper.
A horizontal fluorescent tube which is 3.8 cm in diameter and 120 cm long stands in still air at 1 atm. and 200C. If the surface temperature of the tube is 400C and radiation is neglected, what percentage of power is being dissipated by convection?
Answer
Sign in to view the complete answer paper.
Explain with neat sketch development of velocity boundary layer on hot and cold vertical plate subjected to Natural Convection.
Answer
Sign in to view the complete answer paper.
Discuss the different processes of condensation of vapour on solid surface with suitable diagrams.
Answer
Sign in to view the complete answer paper.
In an oil cooler, oil enters at 160oC. If water entering at 35oC flows parallel to oil, the exit temperature of oil and water are 90oC and 70oC respectively. Determine the exit temperature of oil and water if the two fluids flow in opposite directions. Assume that the flow rates of the two fluids and Uo remain unaltered. What would be the minimum temperature to which oil could be cooled in parallel flow and counter-flow operations?
Answer
Sign in to view the complete answer paper.
What is a gray body? How does its emissivity value will vary for the real surface?
Answer
Sign in to view the complete answer paper.
An enclosure measures 1.5 m × 1.7 m with a height of 2 m. The walls and ceiling are maintained at 250oC and the floor at 130oC. The walls and ceiling have an emissivity of 0.82 and the floor 0.7. Determine the net radiation to the floor.
Answer
Sign in to view the complete answer paper.